P41A-0886 0800h
Water Vapor Diffusion Through Porous Regolith at Mars Environments
Water vapor diffusion is measured through porous media at Mars-like conditions and diffusion coefficients are obtained. A series of homogeneous porous glass disks exhibiting a range of pore sizes (10 to 220 microns) and thicknesses (0.25 to 8 cm) are used in initial experiments to validate the experimental setup and develop a system model. Correction factors to diffusion coefficients for gas-gas diffusion for this experimental geometry are obtained and applied to further samples. The values of porosity and grain/pore size studied cover the range of reasonable physical properties for near-surface Martian regolith. The diffusion coefficients for porous disks and unconsolidated Mars analog materials studied all fall within the range 5 to 15 cm$^2$s$^{-1}$, consistent with theoretical models. Calculated values of tortuosity factor also fall in the expected range of 2 to 5. Implications for ground-ice stability and equilibrium with the Martian climate are presented in the context of previous theoretical investigations.
P41A-0887 0800h
Inferences on the flow dynamics of fluidized ejecta on Mars from topography
Fluidized ejecta on Mars probably flowed along the surface during their final stages of emplacement. Simple continuum flow models can provide inferences on the fluid dynamics responsible for the observed topographic shapes and morphology of these flows. We compare the solution of such a model in cylindrical geometry to the observed topography of fresh ejecta seen in Lunae Planum, Mars. Comparisons are made with ejecta facies that frequently possess an inner flow with a subtle rampart, separated by a moat and a very pronounced distal contiguous rampart. These are classified as multi-layered (MLE) by the Mars Consortium. We consider the fluid dynamics of two flow types that are likely to describe the emplacement of fluidized ejecta: (1) a basal glide flow where displacement is restricted to a small interface at the base of the flow, and (2) a debris or turbidity flow where motion is dependent on flow thickness to 3/2. The basal glide model describes well granular flows and long-run landslides. A debris flow model could explain the presence of boulders observed at the distal edge of pronounced ramparts of many fluidized ejecta that are reminiscent of the boulders that debris flow often transport at their distal edge. We will present continuum flow solutions for three assumptions: (1) conservation of volume, (2) conservation of mass where volume can change through fragmentation, and (3) entrainment and/or deposition during the final stages of emplacement. Solutions where the volume of the flow is conserved indicate that basal glide best describes the topography of the inner region of the flows (prior to the moat) as well as the distal ramparts.
P41A-0888 0800h
Ejecta Curtains and Ground Ice on Mars: Efficiencies of Volatile Release
One of the leading hypotheses that might account for a vigorous hydrologic cycle in the late Hesperian on Mars is that major impact events injected near-surface water into the atmosphere (Segura et al. 2001). In particular, a 50 km diameter impactor is calculated to deposit several meters of "rock rain" globally, potentially triggering the widespread global release of water from ground ice. While the release of volatiles near the impact site is a comparatively simple matter of plotting shock levels as a function of distance, and assuming that target rocks shocked to greater than a given level release all their ice as vapor (Dey, 1989), the potentially greater effect is how efficiently the crater ejecta, which is far more widespread, releases vapor. Near the crater thick sheets of poorly sorted but shock-heated materials plow up the surface at relatively low velocities. Far from the crater the ejecta strike the surface at high velocities but are more thinly spread or confined to discrete patches or jets. Beginning with 1D models, and moving to 2D and eventually 3D models, we examine, using the adaptive-mesh Eulerian hydrocode SAGE (Gisler et al. 2004), the effect of crater ejecta and secondary impacts on the transient volatile budget in the proposed late Hesperian hydrologic cycles. We make use of advanced equations of state, including mixed Alluvium-H20 equation of state in the ice-saturated regolith.
P41A-0889 0800h
Implications of the Anomalous Cratering Record of the South Layered Deposits
The polar layered deposits are among the youngest/most-active surfaces on Mars and play a critical role in the transfer and storage of volatiles on a global scale. The surface exposure ages and modification rates and processes are key to understanding how these deposits respond over time to the independent changes in orbital elements. Through our cratering studies of the south polar layered deposits (SPLD) we have identified over 150 definite craters and characterized their distribution, depths and sizes. There are also three (and possibly four) distinct secondary fields on the SPLD that we have separately characterized. We have utilized the MOC and THEMIS visible datasets and the gridded MOLA products in this research, all referenced to a Mars GIS. Here, we present our findings concerning the isolated background craters on the SPLD which are well removed from those obvious secondary fields and whose morphology is suggestive of a hypervelocity origin. There are anomalous aspects to these SPLD craters. The population of craters greater than 1 km diameter exhibit generally different properties than the population of craters smaller than this. Anomalous and puzzling properties of these populations which must be recognized inorder to infer the exposure ages and modification processes which have characterized these surfaces include: 1. The distribution of craters with diameters > 1km is highly non-uniform with a distinct clustering of these larger craters over about half of the SPLD (especially longitudes 180-240E). Furthermore, that population of craters > 1km diameter is extremely shallow with depths-to-diameter generally less than 0.02. The smaller-sized population, in contrast, is more uniformly distributed over the SPLD and exhibits a full spectrum of depth-to-diameters up to 0.25 These differences in geographic distribution and in morphology imply at least one quite recent resurfacing episode and at least one major unconformity in the lower portion of the SPLD. 2. Although some isolated small craters exhibit unusually low depth-to-diameter ratios, which may indicate they are low velocity secondaries in origin, many others exhibit clear hypervelocity impact morphology, e.g., circular outlines, raised rims, bowl shapes and, in some cases, even ejecta blankets and secondary craters. 3. Size-frequency distributions (SFDs) provide a means to date a surface if the crater production rate is known and follows a predictable power law relationship. The SFD of our SPLD craters however does not follow a predicted power law which may indicate an unmodeled size-dependant modification process. The Log Abundance/ Log Frequency exponent is only - 0.7 to -1.0, well below even the usually accepted "primary" production exponent of -2.0. 4. Our investigations into the northern polar layered deposits reveal far fewer craters than for the SPLD. We have located only 4 impact craters (all smaller than 450 meters across) despite searching a comparable geographic area. Based on the area we have searched so far we estimate that the total population (greater than a few 10s of meters across) will not exceed about 25 impact craters. We will elaborate the above points in more detail and use them to create a Preliminary Chronology of the Polar Layered Deposits.
http://www.gps.caltech.edu/~bcm/AGU-2004-Abstract-Figures.html
P41A-0890 0800h
The Latest Hits: Investigating the Recent Cratering Record on Mars
We have initiated a preliminary study of sub-kilometer craters exhibiting distinct ejecta within 160 MOC-NA images located between $20\deg$W - $120\deg$W longitude and $30\deg$S - $90\deg$N latitude as well as several on the flanks of Olympus Mons. Most of the craters showed distinctly rayed ejecta (49%), while 27% (96) displayed circular, 12% (43) lobate, and 10% (36) exhibited a combination of several blanket types. Two-thirds of the craters showed dark ejecta (64.7%, 230 craters) compared to the surrounding target surface. No preferred elevation was evident from the data, nor was any surface-age preference detected. Crater counts on this population indicate that ejecta retention is a strong function of crater diameter for craters less than ~200m; above this range, the slope of the size-frequency distribution (D-n) approaches the Hartmann isochron of ~1Ma, indicating that ejecta retention is a reasonably reliable indicator of recent crater formation. The low slope of the D-n plot below 200m reveals that surface activity is extremely efficient at obscuring the ejecta around the smallest craters on Mars. This seems reasonable that the ejecta deposits of increasingly smaller craters are themselves thinner and finer grained and are therefore more easily removed or obscured by aeolian activity.
P41A-0891 0800h
History of Major Degradational Events in the Ancient Highlands of Mars: Preliminary Results From Crater Depth and Diameter Measurements
Impact crater depths (d) and diameter (D) were measured for 1398 craters in the diameter range 6 km to over 100 km using the 128th degree MOLA DEM in 10 test locations (Meridiani, Margaritifer, Memnonia, Ma'adin, Promethei, Thyles, S. Argyre, Sirenum, Tempe, and Dueteronilus Regions) in an effort to gain insight into the degradational history of the ancient highlands of Mars. The d/D scatter plots commonly show that Early/Middle Noachian-age terrains located in the mid-latitude (i.e., $\sim$40§S and 40§N) include 3 modes formed by clusters of point that approximately parallel the fresh crater curve, but are displaced relative to each other and have slightly different d/D slopes. The slopes decrease with increased age of the mode. The oldest mode is Early Noachian-age and, most likely, is the result of craters buried by ancient ridged plains materials. The intermediate (in age, relative crater depth, and slope of the d/D distribution) mode is Middle Noachian and is most likely associated with the formation of the valley networks common to these regions. Craters in the third mode are younger than late Noachian in age and include the deepest craters (some have fresh crater d/D), and have the steepest d/D distribution suggesting crater modification by such degradation processes as eolian infilling. In contrast, similar age highland terrains in the high latitudes of Mars (i.e., 40§S to 70§S and 40§N to 55§N) show two modes in their d/D distributions. These are similar to the two oldest modes with the shallowest craters in the mid-latitude regions and, remarkably, indicate that these regions include no craters with fresh d/D values. This suggests either the operation of a process only in the high latitudes of Mars that recently significantly degraded or buried all craters or that the crust of the region is composed of materials that cause craters to initially form relatively shallow and that the recognized fresh crater curve does not apply in these regions.
P41A-0892 0800h
Cratering Evidence For The Age And Thickness Of An Extensive Ice-Rich Mantle In Western Utopia Planitia, Mars
Polygonal features with characteristic dimensions of 100 +/- 30 m, bounded by cracks, are commonly observed on the martian northern plains. These features have previously been attributed to thermal cracking, in direct analogy to ice-wedge polygons in terrestrial polar regions. Polygons were mapped in the northern mid latitudes (30 to 65 N) using all Mars Orbiter Camera (MOC) narrow-angle images (5 m per pixel) from September 1997 through September 2003. Three fourths of MOC images showing polygons are centered in western Utopia Planitia (40 to 50 N; 258 to 288 W). This region, notable for its dense concentration of polygonal terrain, is otherwise non-unique in its mapped geology, topography, gravity, or albedo. Previous authors have suggested that this concentration of polygons indicates the presence of a generally continuous ice-rich mantle. Ice stability models, neutron spectroscopy data, and the common occurrence of thermokarst indicate that the ice is concentrated below 1 m depth and is currently subliming. The MOC images show 687 craters, with diameters between 100 m and 4 km, on polygonal terrain. The size-frequency distribution of these craters larger than 1 km is concordant with the Barlow distribution for craters larger than 8 km in western Utopia, indicating preservation of a late Hesperian crater population. Approximately 20 of the 687 craters on polygonal terrain postdate the adjacent polygonal cracks, indicating Amazonian-age deposition or activation of the ice-rich layer. The size-frequency distribution of craters on polygonal terrain shows a marked deficiency of craters smaller than 1 km, suggestive of mantling. Some such craters with diameters between 460 m and 1.1 km are buried to their rims by polygonal terrain; below this range all rims are buried, and above all rims are exposed. Based on the MOLA-derived relationship between rim height and crater diameter, this range indicates that the ice-rich layer is locally 30 to 40 m thick. These findings are in accord with recent models of obliquity-driven deposition and sublimation of ice-rich mantles in the northern mid latitudes of Mars.
P41A-0893 0800h
A Unique Origin for Mojave Crater?
Williams et al (LPSC 2004; see also Kerr, Science vol. 304, 196 ) recently reported the discovery of a spectacularly eroded, apparently water-carved impact crater on Mars. The ~60 km diameter crater, dubbed "Mojave Crater" for the resemblance of its alluvial fans to alluvial fans seen on Earth, is located on Xanthe Terra, Mars. The crater is rated as late Hesperian or Amazonian, meaning that the impact took place some time in the past 3.5 billion years; there is some evidence that it is not extraordinarily young. Yet the crater seems unique for its size. This crater, and to present knowledge this crater alone, is the obvious source of its own precipitation. Impact by a live, ice-rich comet may provide a unique origin for this unique crater. Impacts by live comets are surprisingly rare in the inner solar system. To make a 60 km crater on Mars requires either a ~6 km diameter nearly isotropic comet (NIC), or an ~10 km diameter Jupiter-family comet (JFC). NICs strike Mars about 3 times in 10 billion if they pass its orbit. The latter happens about 6 times per decade for NICs bigger than 6 km, so that at current rates a NIC makes a 60 km crater on Mars about once every 5 billion years. The typical Mars-crossing JFCs actually hits Mars at a rate of about 8e-11 per comet per year. There are currently 3 known Mars-crossing JFCs that may be big enough to make 60 km craters on Mars (the nearly extinct P/28 Neujmin 1, P/10 Tempel 2, and perhaps P/49 Arend-Rigaux). The rate is about once per 4 billion years. Thus we expect on the order of one 60 km martian impact crater in the past 3.5 Gyr made by a comet. There would be on the order of ten 30 km craters. Preliminary GCM simulations of comet impacts indicate that, while very large impact events generate global climate effects that can lead to precipitation in topographically favored locations anywhere on Mars, on the scale of 60 km craters the impact-generated rainfall becomes localized, such that, to first approximation, a uniquely wet impact might be expected to create uniquely heavy local precipitation on scales comparable to the impact crater.
P41A-0894 0800h
Tomography Study of Shock-Induced Damage Beneath Craters by Normal and Oblique Impacts
Comparisons of laboratory impact craters produced in rock and planetary-scale impact structures, indicate that the observed reductions in elastic wave velocities by shock-induced damage of rock beneath impact craters can be used to constrain the impact history. A series of small-scale normal and oblique impact experiments were conducted on 20x20x15 cm samples of San Marcos granite by a 1.2 km/s, 2 kJ impactor. The resulting largely circular (8 cm in diameter) crater dimensions agrees closely with previous data. By conducting a multiple source-receiver ultrasonic survey of the shocked rock beneath laboratory craters (sampled by 290 ray paths beneath the crater) we have tomographically mapped the in-situ P-wave velocity beneath craters and find measurable damage, as defined by $ > $ 0.1 km/s velocity reduction, are induced to depths of 7 cm beneath the crater for normal impacts. However, oblique impacts produce shallower damage zone ($\sim$ 3 cm deep) that are asymmetric along the plane containing the impact trajectory. The downrange shows more damage than the uprange. Since the extent of the shock-damage region depends on impact velocity and impact energy, the extent of damage in our laboratory impact structures , and we presume also planetary scale impact structures, carries both impact velocity and direction of impact information not previously recognized or sought. Hence damage zone dimensions are expected to constrain planetary impacts parameters. Oblique impacts, where the tracjectory is $\geq$ 15$\deg$ relative to the impacted surface, yields approximately circular craters, can in principle, provide information on impactor trajectory. For planetary impacts, the damage profile, as measured by seismic velocity deficit, beneath craters allow some statistical constraint on impacts produced by low-inclination orbit objects (asteroids and Jupiter-family comets), versus, high-inclination orbit objects (long-period and new comets).
P41A-0895 0800h
Topographic and Remote Sensing Analysis of Explosion Craters in the Nevada Test Site: Application to Mars Hazards
High-resolution LIDAR (LIght Detection and Ranging) data were recently acquired for several explosion craters at the Nevada Test Site (NTS) for the purpose of generating high-resolution (1- to 2-m horizontal posting; 50-cm vertical accuracy) digital elevation models (DEMs). The LIDAR DEMs and ancillary datasets, including ASTER visible-IR images and SIR-C radar images, are to be used in the study of cratering processes. Among the questions we seek to answer are whether these explosion craters can be used as analogs for impact craters on Mars. To this end, we present an error analysis of the LIDAR DEMs and an assessment of their utility in hazard avoidance applications for landers and rovers. We show preliminary data integration results in which we geologically characterize the multisensor signatures of the craters and their ejecta. These results are interpreted in terms of hazard zones defined by different types of ejecta, rock populations, and topographic characteristics (e.g. slope, aspect, relief). From these, we aim to produce a quantitative hazards map for craters in the NTS that can serve as a model for the type of information LIDAR and imagery can provide to a hazard avoidance system. All data and derived products are being manipulated and cataloged in a geographic information system (GIS), which will facilitate the packaging and distribution of materials via the Planetary Data System or other community data repository. Work performed under contract to NASA.
P41A-0896 0800h
Mars-Lab: First Remote Sensing Study of Mineralogy Exposed at Small Mars Analog Craters, Nevada Test Site
Near-surface explosive tests at the Nevada Test Site (NTS) in the Mojave desert created numerous craters that are unique Mars analog sites. Infrared remote sensing is a primary method used to identify minerals on Mars. Small craters expose near-surface composition, weathering processes, and layering. The MarsLab project is the first thermal infrared, remote sensing study of the mineralogy exposed by small terrestrial craters (25-400 m diameter). Small craters are important because they can partially stand-in for drilling, both for more sites than drilling alone can cover, and also when a drilling capability is not otherwise available (e.g., a small rover). On Mars, identification of the minerals on the crater interior wall and in ejecta would uncover currently unknown information on near-surface compositional variations and weathering processes. The Nevada Test Site is a restricted access test facility, managed by the U.S. Department of Energy, in the Mojave desert, approximately 65 miles northwest of Las Vegas. The Mojave desert has long been used for Mars analog studies due to the dry climate. The NTS is particularly valuable because limited public access preserved locations of interest relatively undisturbed. We chose craters in basalt and alluvium substrates. Data sets used include the airborne hyperspectral imager SEBASS (7.5-12.5 microns, 128 bands); Tonka (7.5-12.5 microns, 512 bands), which is the only field spectrometer that raster-scans thermal infrared images like the Mars rover MiniTES; and laboratory spectrometer data sets that cover the full spectral range measured by both the Mars and terrestrial analog instrumentation.
http://www.lpi.usra.edu/science/kirkland